GCN Circular 18123
J. Japelj (U. Ljubljana), C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU) on behalf of a large collaboration report: We observed the position of GRB 150811A (Evans et al. GCN Circ. 18119) with the 1-m McDonald telescope (Fort Davis, US), a member of the Las Cumbres Observatory network. Observations started on August 11 at 06:13:35 UT, i.e., 127 minutes after the GRB trigger time and were done with the r' and i' filters. We detect the optical afterglow (Butler et al. GCN 18120, Zheng et al. GCN 18121, Kuin et al. 18122) with the magnitude of r' = 18.4 +- 0.2 at 132 min after the GRB trigger. Magnitude has been calibrated against nearby USNOB-1 stars.